An armillary sphere is a model astronomical instrument consisting of a set of graduated metal rings, or armillae, representing the great circles of the celestial sphere, such as the celestial equator, the ecliptic and the meridian, arranged around a central sphere or point representing the Earth or, in later heliocentric versions, the Sun, used historically to demonstrate the motions of celestial bodies and to make astronomical measurements and calculations. Instruments of this general type were developed independently in both ancient Greek astronomy, associated with figures such as Eratosthenes and Ptolemy, and in ancient Chinese astronomy, where armillary instruments were built and refined by astronomers over many centuries. Armillary spheres served both as genuine working observational and computational instruments in antiquity and the medieval period and, especially from the Renaissance onward, as prominent teaching and demonstration devices illustrating the structure of the heavens, a role in which elaborately decorated armillary spheres remain recognizable today as symbols of astronomy and exploration.
Facts
Measuresthe positions and paths of celestial objects, modeled as lines of celestial longitude, latitude and other astronomical reference circles 1 Operating PrincipleA framework of graduated rings representing the celestial equator, ecliptic, tropics and meridian is set to a given latitude and turned by hand or winch so that sun and moon pointers trace the real motion of the sky, or, in the Copernican form, of the Earth, letting the user read rising, setting and culmination times off the hour circle. 1 Connections
Precursor To
Astrolabe, Instruments Both are Hellenistic in origin; the planispheric astrolabe (credited to Hipparchus's stereographic projection, c. 150 BC, and developed further through the Islamic Golden Age) is a flat, portable working-out of the three-dimensional celestial model the armillary sphere had already made physical.
Used In
Astronomy, Disciplines The armillary sphere models the celestial sphere's great circles and was a standard demonstration and observational instrument of pre-telescopic astronomy.
Sources
1. Armillary Sphere (Wikipedia)
WikipediaWikipedia, Armillary sphere article, lead section, first sentence
An armillary sphere (variations are known as spherical astrolabe, armilla, or armil) is a model of objects in the sky (on the celestial sphere), consisting of a spherical framework of rings, centered on Earth or the Sun, that represent lines of celestial longitude and latitude and other astronomically important features, such as the ecliptic.
Wikipedia, Armillary sphere article, Description and use section, closing sentence
And so, by this construction, the machine is equally fitted to show either the real motion of the earth, or the apparent motion of the heavens.
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